ML4812IP datasheet(8/16 Pages) MICRO-LINEAR

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ML4812

The recommended maximum duty cycle is 95% at

100KHz to allow time for the input inductor to dump its

energy to the output capacitors. For example, if: VOUT =

380V and DON (max) = 0.95, then substituting in (3)

yields VINDRY = 20V. The effect of drying out is an

increase in distortion at low voltages.

For a given output power, the instantaneous value of the

input current is a function of the input sinusoidal voltage

waveform, i.e. as the input voltage sweeps from zero volts

to a maximum value equal to its peak so does the current.

The load of the power factor regulator is usually a

switching power supply which is essentially a constant

power load. As a result, an increase in the input voltage

will be offset by a decrease in the input current.

By combining the ideas set forth above, some ground

rules can be obtained for the selection and design of the

input inductor:

Step 1: Find minimum operating current.

PEAK

(min)

.(min)

(max)

1414

(4)

VIN(max) = 260V

PIN(min) = 50W

then:

IIN(min)PEAK = 0.272A

Step 2: Choose a minimum current at which point the

inductor current will be on the verge of drying

out. For this example 40% of the peak current

found in step 1 was chosen.

then:

ILDRY = 100mA

Step 3: The value of the inductance can now be found

using previously calculated data.

KHz

INDRY

LDRY

OSC

0 95

100

(max)

(5)

The inductor can be allowed to decrease in value when

the current sweeps from minimum to maximum value.

This allows the use of smaller core sizes. The only

requirement is that the ramp compensation must be

adequate for the lower inductance value of the core so

that there is adequate compensation at high current.

Step 4: The presence of the ramp compensation will

change the dry out point, but the value found

above can be considered a good starting point.

Based on the amount of power factor correction

the above value of L1 can be optimized after a

few iterations.

Gapped Ferrites, Molypermalloy, and Powdered Iron

cores are typical choices for core material. The core

material selected should have a high saturation point and

acceptable losses at the operating frequency.

One ferrite core that is suitable at around 200W is the

#4119PL00-3C8 made by Philips Components

(Ferroxcube). This ungapped core will require a total gap

of 0.180" for this application.

OSCILLATOR COMPONENT SELECTION

The oscillator timing components can be calculated by

using the following expression:

OSC

136

(6)

For example:

Step 1: At 100kHz with 95% duty cycle TOFF = 500ns

calculate CT using the following formula:

OFF

DIS

OSC

= 1000

(7)

Step 2: Calculate the required value of the timing

resistor.

KHz

k choose R

OSC

136

100

1000

136

(8)

TYPICAL APPLICATIONS (Continued)

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